Blanking circuit for electron multiplier



Jan. 15, 1957 J A. HENDERSON 2,777,948

BLANKING CIRCUIT FOR ELECTRON MULTIPLIER 2 Sheets-Sheet 1 Filed June 14, 1951 INVENTOR. d A. HENDERSON ATTORNEY Jan. 15, 1957 J A. HENDERSON 2,777,943

BLANKING CIRCUIT FOR ELECTRON MULTIPLIER Filed June 14. 1951 2 Sheets-Sheet 2 ATTORNEY United States Patent BLANKING CIRCUIT FGR ELECTRON MULTIPLIER J Alvin Henderson, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, Fort Wayne, lnd., a corporation of Indiana Application J1me 14, 1951, Serial No. 231,627

8 Claims. (Cl. 250-27) This invention relates to multiplier blanking circuits, and more particularly to circuits for establishing black level in the circuit of an electron multiplier, by applying blanking pulses to the secondary electron emission electrodes of the multiplier.

Electron multipliers, as known, comprise amplifiers which secure the amplification of signals by successively applying signal modulated electrons to secondary electron emissive electrodes. Such multipliers among other uses are quite commonly used in image analyzing tubes such as image dissectors to secure a predetermined amplification of the image signals from such tubes prior to the application of its video signals to external circuits. In the transmission of image signals it is necessary that certain control pulses be included for the purpose of synchronizing operation of equipment at the receiver. These control pulses may, for example, be horizontal synchronizing pulses, vertical synchronizing pulses or color switching pulses. Such signals are inserted into the image signal wave after providing a blanking or cutting off of the signal in external circuit elements such as amplifier tubes or the like.

It is desirable in some cases to provide for the insertion of certain of these blanking pulses directly into the signal prior to application to external circuits. One reason for this is to establish black level before external amplification so that a clamp circuit occurring later in the amplification chain can re-establish all low frequency information, eliminate microphonics and hum-all of which problems occur in the low level amplifier stages. It has been proposed to accomplish this by applying a positive voltage to the last electron multiplier stage, or secondary emissive electrode, so that this electrode will serve essentially as a collector element and the energy being amplified will, therefore, not be applied to the output target or collector electrode. However, in such cases it is found that a certain residual signal is still impressed on this output electrode due in part to its capacitive coupling with the last electron multiplier stage and in some cases due to direct embodiment of electrons from the penultimate electron multiplier stage, because of the geometry or structural arrangement of the multiplier elements.

It is an object of this invention to provide a system for inserting control pulses such as blanking pulses into a signal being amplified by an electron multiplier by applying positive potential pulses to one of the secondary emissive electrodes of the multiplier, and to compensate for the capacitive coupling by applying simultaneously a pulse of opposite polarity to a subsequent emissive electrode or to the collector electrode.

According to a feature of this invention there is provided a coupling arrangement between a blanking pulse source and the electron multiplier whereby impulses from the source may be applied in positive polarity to the last electron multiplier stage, or secondary electron emissive electrode, or to the next to the last secondary emissive electrode and simultaneously pulses from the same source may be applied in negative compensating relationship to the output collector electrode, or to the last secondary emissive electrode of the multiplier.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which Figure 1 is a diagrammatic illustration of a portion of an electron multiplier and the schematic circuit for operation in accordance with this invention, and

Figure 2 is a diagrammatic illustration of a portion of an electron multiplier and an alternative blanking circuit in accordance with this invention.

Turning first to Fig. 1 there is shown at 1 an electron multiplier which may comprise a plurality of secondary electron emissive electrodes 2, 3, 4, 5, 6 and 7 and an output or collector electrode 8. Secondary emissive electrodes 2 through 6 are coupled across various resistors 9 of a voltage dividing network, supplied at one end with a negative voltage so that the successive secondary emissive electrodes are succeedingly of higher positive potential. Thus as a stream of electrons from any given source is impinged upon electrode 2 the secondary electrons emitted will be attracted to the succeeeding multiplier stages producing an amplification of an applied signal energy. The output from electrode 7 thus impinges on electrode 8, shown as coupled to ground, or a common reference potential, which itself is normally positive with respect to electrode 7 and the drop in voltage caused by this signal energy traversing resistor 10 may be communicated by the output terminal 11 to the desired load circuit. Bridged across the separate resistors of electrodes 2 through 6 are provided voltage regulator gas discharge tubes 12 which serve to assure uniform voltage drop between these electrodes. Electrode 7 is coupled through additional resistors 13 and 14 to B+ and the voltage drop between this electrode and electrode 6 is controlled by voltage regulating gas discharge tube 15.

A source of control pulses or blanking pulses is shown at 16 in the output of which is provided a train of control or blanking pulses which are negative in potential as shown at 17. The electron discharge device 18 is provided having its anode 19 connected to electrode 7 and its cathode 20 connected over a resistor 21 to the next preceding electrode 6. The voltage drope between electrodes 7 and 6 is preferably suliicient so as normally to maintain tube 18 conducting. Thus in the absence of application of blanking pulses proper voltages are supplied to all of the electron multiplier electrodes to provide the normal amplification action. When negative pulses 17, however, are applied to the grid 22 of tube 18, the tube may be driven to cut-ofi producing a relatively high amplitude positive output pulse 18a which is applied directly to electrode 7. This positive pulse is preferably of sufficient amplitude to bring electrode 7 to a potential at least as high as collector electrode 8 so that the secondary electrons striking on electrode 7 will tend to be held there and will thus not reach the output electrode 8. Due however to the proximity of electrode 8 to electrode 7 there is considerable capacitive effect and consequently a tendency to feed certain of the energy through terminal 11. This will generally be in the form of positive and negative peaks or spikes at the initiation and termination of the pulse 18a. In order to compensate for this action, negative pulse 17 is simultaneously applied through a variable capacitor 24 to collector electrode 8. Capacitor 24 is preferably adjusted to simulate and compensate the capacitance between electrode 8 and electrode 7 so that there is applied to this electrode a negative pulse of substantially the same form but of opposite polarity to that which would be applied through the inherent coupling between the electrodes. Resistor 13 is equal to resistor 21 so that equal amplitude but opposite polarity pulses are developed The coupling point between resistor 23 and resistor 21 may be properly adjusted with respect to the other resistors of the system to provide the desired static voltage on the number 7 dynode.

It will be clear from the above description that at output terminal 11 the normal signal output will appear until such time as blanking pulses from 16 are applied to electrodes 7 and 8. When these pulses are applied, the output through electrode 8 will be completely cut-elf so that the desired control pulse eiiect will be inserted into the output energy at terminal 11.

The circuit arrangement described above is quite satisfactory but because of the critical adjustment required in connection with the compensating pulse application, it is desirable to minimize stray capacity efiects. In Fig. 2 is illustrated an alternative circuit arrangement incorporating the features of the invention which does not require such a high degree of neutralization. In this figure only four secondary emissive electrodes 4 through 7 have been illustrated together with a collector electrode 8 similar to those shown in Fig. l. Electrodes 4 and are supplied with the desired successively increasing positive potential. Electrode '7 is connected through tunable inductor 26 and resistor 27 to ground so as to have the proper positive potential with respect to electrode 6, and electrode 8 is connected to reference potential through output resistor 28 so that the signal output may appear at output terminal 29. In addition, it may be noted that electrode 6 is also coupled through a tunable inductor 30 and resistor 25 to ground.

A blanking pulse source 16 is provided as in the case of Fig. 1, the output pulse 31 of which may be applied over a coupling condenser 32 to the control grid 33 of a tube 34. In the illustrated embodiment the pulses 31 are shown as positive so that negative pulses are produced at anode 35 of tube 34 and positive pulses at the cathode 36 of this tube. In the output of cathode 36 are provided resistors 37 and 38 and grid bia'ssing resistor 39 is shown connected intermediate these resistors and the grid 33. A by-pass condenser 4t) may be provided between the anode supply lead and ground as shown.

Positive output pulses from the cathode resistor may be fed over a coupling condenser41 to the control grid of a tube 42, the anode of which is coupled through inductance 26 to the last secondary emissive electrode '7. The positive pulse 43, which is equal in amplitude but opposite in phase to pulse 47since resistor 37 plus resistor 38 equals resistor 50, applied to the grid of tube 42 serves to apply a negative pulse 44 to electrode 7.

The anode 35 of tube 34 is coupled over coupling condenser 45 to the input grid of a second control tube 46, the anode of which is coupled through inductance element 30 to electrode 6. The negative pulse 47 from anode 35 will, therefore, appear as positive pulse 48 applied to electrode 6. The cathodes of tubes 42 and 46 are maintained at a regulated voltage by means of regulator tubes 49 connected between these cathodes and ground.

It will be clear from the description of this circuit that with each blanking pulse from source 16 there will be applied a positive pulse to electrode 6. This positive pulse is of high amplitude and tends to render electrode 6 more positive than the succeeding electrodes 7 and 3 so that it acts as a collector electrode. However, due to the geometry of the tube some of the electrons from 5 may be impinged upon electrode 7 so that the signal would not be completely cut-oil. The negative pulse however applied to electrode 7 simultaneously with the application of the positive pulses to electrode 6 causes this electrode 7 to become negative tending to repel any elec-' trons back toward electrode 6 so that they will not reach output electrode 8. Because of the capacitive coupling between collector electrode 8 and electrode 7 there still may be residual peak pulses fed through to output terminal 29 although they would be greatly minimised. If this effect is found to be undesirable a clamping voltage may be applied to succeeding tube circuits so as to prevent the positive and negative peaks, due to the capacitive coupling, from atlecting further circuits.

While this invention has been described in connection with some specific embodiments, it is clear that these particular forms are only given by way of illustration. The electron multiplier may be of any type and not necessarily of the box type as illustrated. Furthermore, tubes other than triod'es may be provided and the various control tube arrangements may be connected in circuit in a diiferent manner, it only being necessary to procure the desired amplitude and polarity of the applied blanking pulses.

While -I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

l. A circuit for inserting a pulse signal into a signal energy wave comprising an electron multiplier amplifier for said signal wave, having a plurality of electrodes including a collector electrode and a plurality of secondary emissive electrodes, an output circuit coupled to said collector electrode, means for normally biassing said electrodes to successively greater positive potentials, a signal source, first means for applying a signal from said source to one of said secondary emissive electrodes in positive polarity only, and second means for simultaneously applying a signal from said source to a succeeding one of said secondary emissive electrodes in negative polarity only whereby said signal energy wave will be blocked from said output circuit for the duration of said source signal.

2. A system for combining different signals into a single signal train comprising an electron discharge device having a plurality of electrodes between which signal electrons flow in successive order, first means for applying a first signal of positive polarity only to one of said electrodes, and second means for simultaneously applying a second signal of negative polarity only to a succeeding one of said electrodes, said first and second means producing signals of such magnitude which conjointly serve to interrupt substantially completely said flow of electrons.

3. A system for combining different signals into a single signal train comprising an electron discharge device having a plurality of electrodes between which signal electrons flow in successive order, first means for applying a first unidirectional signal of positive polarity only to one of said electrodes, and second means for simultaneously applying a second unidirectional signal of negative polarity only to a succeeding one of said electrodes, said first and second signals conjointly serving to interrupt substantially completely said flow of electrons.

4. A circuit according to claim 2, wherein said first means comprises a coupling to the next to the last electrode, and said second means comprises a coupling to the last electrode.

5. A circuit according to claim 2, wherein said last named means comprises a condenser for compensating the capacitive coupling inherent between said one electrode and said succeeding electrode.

6. A circuit according to claim 1, wherein said first means comprises a coupling to the second from the last of said secondary emissive electrode, and said second means comprises a coupling to the last of said secondary emissive electrodes.

7. An electron multiplier and signal pulse inserting circuit comprising an electron multiplier having a plurality of successively positioned secondary emissive electrodes, and an output collector electrode, means for normally providing successively higher positive potentials to said electrodes, whereby successively amplified energy may be applied to said output electrode, a source of signal pulses of negative polarity, an electron discharge device having an anode, cathode and grid having its anode connected to the last of said secondary emissive electrodes, means for applying said signal pulses to the grid of said electron discharge device, whereby a positive potential only will be applied to said last secondary emissive electrode, and a resistive and capacitive coupling between the cathode of said electron discharge device and said output electrode for simultaneously applying a negative pulse only to said output electrode to neutralize the inherent capacity between it and said secondary emissive electrode, whereby a signal pulse will be produced at said output electrode.

8. A circuit for inserting a pulse signal into a signal energy wave comprising an electron multiplier having a plurality of secondary emissive electrodes, and an output collector electrode, means for normally providing successively higher positive potentials to said electrodes whereby said wave is successively amplified and applied to said output electrode, a source of signal pulses of a predetermined polarity, electron discharge means for applying pulses from said source to the last secondary emissive electrode in negative polarity and to the next to the last of said secondary emissive electrodes in positive polarity, whereby said signal energy is substantially cut-off at said output electrode to provide said signal pulse output.

References Cited in the file of this patent UNITED STATES PATENTS 

